1. Field of the Invention
The present invention relates to a piezoelectric transformer inverter for converting direct current voltage into alternating voltage by using a piezoelectric transformer. More specifically, it relates to a piezoelectric transformer inverter for lighting a cold cathode tube used for back-lighting of a crystal-liquid display panel.
2. Description of the Related Art
Recently, as a display of a portable information-processing device, such as a mobile phone or a notebook-size personal computer, a liquid-crystal display with back-lighting has been used. As a light source of back-lighting, a fluorescent tube such as a cold cathode tube is used. In order to light the fluorescent tube, high alternating voltage needs to be applied. In addition, as an input power supply of a portable information-processing device, such as a notebook-size personal computer, both a battery and an AC adapter are ordinarily used together. Such back-lighting requires a fluorescent-tube-lighting system, such as a DC/AC inverter for converting low direct current voltage supplied from an input power supply into high alternating voltage capable of lighting the fluorescent tube.
Recently, for use in lighting a fluorescent-tube, a piezoelectric transformer inverter incorporating a smaller type of piezoelectric transformer than an electromagnetic transformer has been under development. For such use, a piezoelectric transformer inverter is required to have the following performance capabilities:
(1) the ability to perform a variable control of the tube current of a cold cathode tube so as to adjust the luminance of a liquid-crystal display panel; PA1 (2) a wide input-voltage range so as to be driven either by batteries or a battery charger; and PA1 (3) a high efficiency to prolong the operating time.
Before describing conventional piezoelectric inverters, a Rosen-type piezoelectric transformer, which is typically used in such systems, will be described. As shown in FIG. 1, there is provided a Rosen-type piezoelectric transformer 1, in which in one-side region in the length direction, a primary electrode 3 is formed on both front-and-back main surfaces of a piezoelectric substrate 2 made of a piezoelectric ceramic material, and the piezoelectric substrate 2 is polarized in a direction perpendicular to both of the primary electrodes 3 (i.e. in the thickness direction of the piezoelectric substrate 2). A secondary electrode 4 is formed on an end face of the piezoelectric substrate 2, and the region on which the secondary electrode 4 is present is polarized in the length direction.
In the Rosen-type piezoelectric transformer 1, when alternating voltage by an input power supply 5 is applied between the primary electrodes 3, the voltage is converted into mechanical distortion. This distortion excites oscillation in the length direction, and the mechanical oscillation is again converted into electrical oscillation to be taken out from the secondary electrode 4 so as to perform a transformer step-up function, whereby the stepped-up voltage is applied to a load 6 (a fluorescent tube).
FIGS. 2(a), (b), (c), (d), and (e) respectively show for the Rosen-type piezoelectric transformer 1, the step-up ratio, the conversion efficiency, the input/output phase difference, and the frequency characteristics of the input impedance, when the load resistance R.sub.L is 1M, 100 k, and 20 k, respectively. In general, the peak of the step-up ratio (=output voltage/input voltage) of the piezoelectric transformer 1 is in the proximity of a resonance frequency fr (=a frequency at which input impedance is the minimum), and an input/output phase difference (=a difference between the phase of output voltage and the phase of input voltage) inverts from 0.degree. to 180.degree. in the proximity of the peak of the step-up ratio. In addition, the peak of conversion efficiency is between the resonance frequency fr and an anti-resonance frequency fa (=a frequency at which input impedance is the maximum). The frequency at which conversion efficiency is at peak is indicated by fd. Furthermore, in FIG. 2, the resonance frequency fr, the anti-resonance frequency fa, and the frequency fd at which conversion efficiency is at peak, are equivalent to those in a case in which load resistance is 100 k.
(First Conventional Embodiment)
Next, a description will be given of a conventional piezoelectric transformer inverter. In a piezoelectric transformer inverter used as back-lighting of a liquid crystal display panel, as described above, variable control of load current (tube current of a cold cathode tube) needs to be performed so as to adjust the luminance of the liquid crystal display panel (the aforementioned requirement (1)). A piezoelectric transformer inverter to meet the requirement, for example, is disclosed in Japanese Unexamined Patent Publication No. 6-167694. As shown in FIG. 3, this takes advantage of changes in the step-up ratio of a piezoelectric transformer as a function of a driving frequency. When control is performed on the higher-frequency side of the peak of the step-up ratio, if a smaller load current is desired, the driving frequency is set to be higher and the step-ratio is set to be smaller, whereas in case a larger load current is desired, the driving frequency is set to be lower and the step-ratio is set to be larger, so that the load current is controlled to be constant. A piezoelectric transformer inverter of such a system has been widely used due to its simple method for controlling load current.
In the piezoelectric transformer inverter of such a system, however, in case a power supply voltage is high or load current is set to be smaller, it is driven in a frequency region of low conversion efficiency, which greatly deviates from the proximity of a resonance point fr or anti-resonance point fa of the piezoelectric transformer, whereby there is a problem of significantly low efficiency, (that is, this does not meet the aforementioned requirement (3)).
(Second Conventional Embodiment)
In order to solve these problems, there are some methods in which high efficient drive is performed in the proximity region of a frequency fd of a conversion efficiency peak by controlling a piezoelectric transformer so that a phase difference in piezoelectric-transformer driving signals is maintained constant, and load current is controlled to be constant by changing the ON-Duty of a switching device. A piezoelectric transformer inverter having such a system has been disclosed in Japanese Unexamined Patent Publication No. 55-98881, Japanese Unexamined Patent Publication No. 1-293170, Japanese Unexamined Patent Publication No. 9-237684, and Japanese Unexamined Patent Publication No. 9-135573, etc.
A piezoelectric transformer inverter 11 as disclosed in Japanese Unexamined Patent Publication 55-98881 is shown in FIG. 4. A drive oscillation transformer 12 drives a piezoelectric transformer 13, and a drive circuit 14 and push-pull transistors 15 and 16 perform a push-pull switching driving with respect to the transformer 12. After an output voltage V.sub.0 of the piezoelectric transformer inverter 11 is divided by voltage-divider resistors 17 and 18, it is compared with a reference voltage E of a direct-current power supply 20 to be amplified by an amplifier 19, and then it is input to one of the inputs of a comparator 21 as an error output.
A phase detector 22 detects the input voltage and the input current of the piezoelectric transformer 13 and converts the voltage-current phase difference into voltage to input to a voltage-controlled oscillator 23. In the voltage-controlled oscillator 23, the frequency changes according to the output voltage of the phase detector 22, and the output of the voltage-controlled oscillator 23 is supplied to the other input of the comparator 21. A pulse output is conveyed from the comparator 21 to the drive circuit 14 corresponding to an error level. In other words, when a voltage output V.sub.o is lowered, the output of the amplifier 19 is lowered and the drive-pulse width of the drive oscillation transformer 12 is increased. As a result, on a secondary side of the drive oscillation transformer 12, voltage of a basic wave component corresponding to an oscillation frequency is heightened and input current of the piezoelectric transformer 13 is increased to raise the output. This system can improve output stability with respect to load fluctuations.
In order to use a piezoelectric transformer efficiently, it needs to be driven at a frequency coinciding with the resonant frequency fr of the piezoelectric transformer. In addition, it is known that the phase difference between input current and input voltage of the piezoelectric transformer deviates by 90.degree. at a resonance point. Thus, in the piezoelectric transformer inverter 11, a phase difference between the input current and the input voltage of piezoelectric transformer 13 is controlled in such a manner that the phase difference between them is 90.degree., as described above, in which control is performed so as to make the output voltage (conversion efficiency) of the piezoelectric transformer 13 maximum, as shown in FIGS. 5(a) and (b).
In the piezoelectric transformer inverter 11, however, a resistor 24 for detecting input current is used, in which input current of the piezoelectric transformer 13 is larger than output current of the same, so that there is a problem in which a loss due to the resistor 24 for detecting input current is large. Furthermore, in this system, since the piezoelectric transformer 13 is driven by rectangular waves, a loss associated with electrical charging/discharging of the piezoelectric transformer input capacity is large, leading to difficulty in achieving high efficiency.
(Third Conventional Embodiment)
Next, FIG. 6 shows a system disclosed in Japanese Unexamined Patent Publication No. 1-293170. Although this is not a drive circuit of a piezoelectric transformer, a phase difference between the input voltage V.sub.l and the input current I.sub.l of a piezoelectric vibrator 31 is maintained constant to achieve high efficiency. In this conventional embodiment, as shown in FIG. 6, the input voltage V.sub.l and the input current I.sub.l of the piezoelectric vibrator 31 are compared by a phase comparison unit 32, and a control oscillator 33 controls a switching device 34 to make the phase of the input voltage V.sub.l the same as that of the input current I.sub.l, whereby the piezoelectric vibrator 31 is driven at a resonance frequency.
However, regarding the attempt of using such a circuit and a controlling system in a piezoelectric transformer inverter, there is difficulty in using them due to a difference between input phase characteristics stemming from the difference in load resistance values. In addition, in the circuit and the controlling system, a step-up electromagnetic transformer 35 is used in the front stage of the piezoelectric vibrator 31. Because of the relatively large size of such a transformer, the circuit and the controlling system are not suitable for lighting a cold cathode tube, which is required to have a low height and a reduced size.
(Fourth Conventional Embodiment)
FIG. 7 shows a structure of a piezoelectric transformer inverter 41 disclosed in Japanese Unexamined Patent Publication No. 9-237684. In the piezoelectric transformer inverter 41, a phase difference between the input voltage and the output voltage (measured as a midpoint voltage of two voltage-divider resistors 43 and 44) of a piezoelectric transformer 42 is detected by a phase-difference detection circuit 45, and according to the phase difference, a control circuit 46 controls the oscillation frequency of a voltage-controlled oscillation circuit 47, whereby the driving frequency of input voltage applied to the piezoelectric transformer 42 from a transformer-driving unit 48 is controlled so that the phase difference between the input voltage and the output voltage of the piezoelectric transformer 42 is 90.degree..
However, in this system, since the phase difference between the input voltage and the output voltage at the step-up peak point is not 90.degree. under the condition of practical use in which a cold cathode tube 49 is lit (The Institute of Electronics, Information and Communication Engineers: Technical Report US95-22, EMD95-18, and CPM95-30, appropriate control cannot be performed. Furthermore, since the piezoelectric transformer 42 is driven by rectangular waves in this conventional system, there is a problem in which a loss associated with charging/discharging of the piezoelectric-transformer input capacity is large.
(Fifth Conventional Embodiment)
An output current phase differences and a conversion efficiency of a piezoelectric transformer inverter disclosed in Japanese Unexamined Patent Publication No. 9-135573, are shown in FIG. 8. The duty control of the piezoelectric transformer of this embodiment is permitted only in the case in which the phase difference is in a constant phase range (P1-P2), whereas the duty control is stopped for a while in the case in which the phase deviates from the constant phase range P1-P2 due to some disturbance.
Although such a controlling system can be achieved if a micro-computer or the like is used, since the size of the resultant circuit is large, product cost is increased and, accordingly, it is not practical in terms of cost.
(Sixth Conventional Embodiment)
Furthermore, when input voltage is high or load current is reduced to be smaller, a piezoelectric transformer is driven in a frequency region of low conversion efficiency, which greatly deviates from a resonance point fr and an anti-resonance point fa of the piezoelectric transformer. As another method for solving the problem in which there is a significant efficiency reduction, for example, there is a method for performing self-exciting oscillation of a piezoelectric transformer at a resonance frequency, as disclosed in Japanese Unexamined Patent Publication No. 7-162052, Japanese Unexamined Patent Publication No. 8-47265, etc., and a method for reducing frequency control width to suppress conversion efficiency reduction by using frequency control as the final adjustment of load current, while changing the ON-Duty of a switching device according to the input voltage, as disclosed in Japanese Unexamined Patent Publication No. 9-51681.
In a piezoelectric transformer inverter 51 disclosed in Japanese Unexamined Patent Publication No. 7-162052, as shown in FIG. 9, an LC resonance circuit 53 is disposed in the piezoelectric-transformer input stage, and the output voltage of the piezoelectric transformer 52 is divided by voltage-divider resistors 54 and 55 to give feedback to the input by a feedback circuit 56 so as to perform self-exciting oscillation. In this piezoelectric transformer inverter 51, since the LC resonance circuit 53 is disposed in the piezoelectric-transformer input stage, there is no loss associated with charging/discharging of the piezoelectric-transformer input capacity.
However, owing to the reason below, this system has a problem in which the piezoelectric transformer 52 cannot be driven at a frequency fd of conversion efficiency peak. In other words, as shown in Japanese Unexamined Patent Publication No. 52-45013, although the step-up ratio by the piezoelectric transformer input 52 itself has a mountainous or single hump form, when the LC resonance circuit 53 is disposed in the piezoelectric-transformer input, the frequency characteristics of the piezoelectric-transformer input voltage is in the form of double-humps as shown in FIG. 10(a). As a result, the frequency characteristics of the piezoelectric-transformer output voltage are also of double-hump form as shown in FIG. 10(b). Since oscillation is sustained at a frequency at which a feedback gain is maximum in the self-exciting oscillation circuit, an operating frequency is equivalent to one of the two humps (a peak) of the piezoelectric-transformer output voltage. However, since the frequency fd of the conversion efficiency peak is a frequency at a bottom between the double humps, it is understood that the piezoelectric transformer is driven at a frequency with poor efficiency in the conventional embodiment shown in FIG. 9.
A simple description will be given of the reason that an LC resonance circuit creates the double-hump form of the piezoelectric-transformer input current. As seen in the electrical characteristics of the piezoelectric transformer shown in FIG. 2, in a region apart from the frequency fd of a conversion efficiency peak, the input phase is approximately -90.degree., namely, it shows capacitance. Consequently, the Q value of the LC resonance circuit including the input impedance of the piezoelectric transformer is increased, so that the input voltage of the piezoelectric transformer is stepped up. Meanwhile, the input phase is close to 0.degree. at a frequency in the proximity of a frequency fd of conversion efficiency peak. That is, the Q value of the LC resonance circuit is lowered and a step-up operation by LC resonance is reduced, so that the input voltage of the piezoelectric transformer is lowered. As a result, the input voltage is reduced at a frequency fd of conversion efficiency peak, whereas the input voltage is increased on both sides of the peak frequency so as to form double humps.
(Seventh Conventional Embodiment)
Next, FIG. 11 shows a piezoelectric transformer inverter 61. In this piezoelectric transformer inverter 61, a switching circuit 66 of a full-bridge structure, including four switching devices 62, 63, 64, and 65, is connected between primary electrodes 68 of the piezoelectric transformer 67 to provide the step-up ratio, while the output voltage is divided by a load 69 and a resistor 70 and feedback of the divided voltage is supplied to the input to perform self-exciting oscillation. In this system, since the frequency characteristics of the output voltage are not in the form of double-humps as in the case of Japanese Unexamined Patent Publication No. 7-162052 (the sixth conventional embodiment), the transformer can be driven in the proximity of a resonance frequency with the highest gain.
However, in this piezoelectric transformer inverter 61, since the piezoelectric transformer 67 is driven by switching on/off the switching circuit 66 by rectangular waves output from a drive circuit 71, there is a problem of loss associated with charging/discharging of the piezoelectric-transformer input capacity.
(Eighth Conventional Embodiment)
FIG. 12 shows a piezoelectric transformer inverter 81 disclosed in Japanese Unexamined Patent Publication No. 9-51681. In the piezoelectric transformer inverter 81, the ON-Duty of a switching circuit 84, in which two switching devices 82 and 83 are made into a half-bridge, is controlled according to a power supply voltage V.sub.CC. In addition, the output voltage of the piezoelectric transformer 85 is detected by a detection circuit 86 to convert into a frequency by a V-f conversion circuit 87 so as to control a drive circuit 88, whereby frequency-control of the output voltage is performed. With the piezoelectric transformer inverter 81, since input-voltage fluctuations can be absorbed by the ON-Duty, the workload in frequency control is lightened and thereby results in gaining of an advantage in which the frequency-fluctuation width associated with output-voltage control is not increased.
In this conventional embodiment, however, since the LC filter 89 is disposed in the input stage of the piezoelectric transformer 85, as explained in referring to FIG. 10, the frequency characteristics of the piezoelectric-transformer output voltage are in the form of double-humps, in which frequency control is actually very difficult. For instance, when the piezoelectric transformer 85 is driven at a frequency fd of conversion efficiency peak, if the inverter output is increased due to some disturbance, the piezoelectric transformer inverter 81 tries to lower the piezoelectric-transformer output voltage by increasing the driving frequency. However, in order to lower the step-up ratio of the piezoelectric transformer 85, it is necessary to go beyond one of the two humps (peaks) of the output-voltage frequency characteristics, whereby the driving frequency greatly increases, so that the operation becomes very unstable.